Dear Emuna, My wife has gained weight and I no longer find her attractive. First of all, I hope that you and any other men that feel that way are wise enough not to tell your wives!
You should reassure her that she is always beautiful to you. If you say it often enough, you may come to remember it and believe it. Thirdly, our sages recommend that intimacy occur in the dark. We should try to dress in ways that are flattering and to “spruce up” for our spouses.
And, knowing that she is making the EFFORT to be PHYSICALLY attractive to him can -- of itself -- increase HIS desire for her.
If sages recognize that a spouse can become "undesirable", maybe you should, as well.
You keep your yourself clean and you treat your wife kindly. When will the frum world get over the myth that looks don't matter.
You make sure she has household help so she has plenty of time to go to the gym and the tennis club and keep herself in shape. They do, and pretending they don't can make for a lot of problems.